Toxoplasma gondii is a protozoan parasite that poses a significant risk to AIDS patients and is listed by NIAID as a Category B Priority Pathogen for biodefense. Fundamental to the pathogenic process is the ability of the parasite to develop into a latent tissue cyst that can re-emerge as a rapidly growing form upon impairment of immunity. Increased understanding of this developmental process will provide new opportunities for therapeutic intervention. The work in our first award period focused on GCN5 histone acetyltransferases (HATs) and led to the discovery that histone acetylation correlates with gene expression pertinent to Toxoplasma development. We have taken genetic approaches to define the roles of two distinct GCN5-family HATs we have characterized in Toxoplasma (TgGCN5-A and -B). We have found that the loss of TgGCN5 impairs the ability of Toxoplasma to up-regulate key developmentally expressed genes. We have also determined that some of the proteins interacting with TgGCN5 in a yeast two-hybrid screen have domains typical of transcriptional regulators; this is significant because the lack of DNA-binding transcription factors detectable in the Toxoplasma genome has hampered efforts to understand how gene expression is regulated. How Toxoplasma regulates transcription to grow effectively during the acute stage, and develop into a latent cyst during stress, represent major gaps in our knowledge that hinder our ability to fight the opportunistic infection. The data generated by the first award allow us to focus this renewal on defining mechanisms of GCN5- mediated gene regulation in the context of parasite development, which underlies pathogenesis. We hypothesize that the TgGCN5 HATs play key roles in Toxoplasma pathogenesis by forming distinct complexes with novel proteins that regulate developmental gene expression. Our specific aims include (1) Determine the roles of TgGCN5 HATs in pathogenesis; (2) Elucidate differences in TgGCN5 complexes during Toxoplasma development; (3) Define the mechanism by which each TgGCN5 is recruited to target gene promoters to coordinate developmental gene expression. The proposed research capitalizes on the novel reagents, techniques, and transgenic parasites that we have developed. The data generated will illuminate the mechanism behind developmental transitions in Toxoplasma that are responsible for disease progression, thus exposing novel points of therapeutic intervention.